Photosynthesis

Ch 8

Environment as a System

• Observations:• Energy flows

• from sun to earth

• Matter cycles• within the four

spheres of earth

• Energy - the capacity to do work• Matter - everything that takes up space and has mass

Energy QualityLow Quality Energy

• Diffused, dispersed, or low in temperature• Difficult to gather and use for productive purposes• Example: heat stored in the oceans

High Quality Energy• Intense, concentrated, or high in temperature• Useful in carrying out work• Example: high-voltage electrical energy

Many of our most common energy sources are low-quality and must be concentrated or transformed into high-quality sources before they are useful to us.

The Energy “Rules”

• The first law of thermodynamics • energy cannot be created or destroyed, but can just

be transformed• The second law of thermodynamics

• when energy is transformed from one kind to another, it is degraded; thus less energy is available to do work.

• recognizes the principle of entropy - the tendency of all natural systems to go from a state of order toward a state of increasing disorder

• Some energy transformations are more efficient than others, but none are 100% efficient.

The Carbon Cycle

2-5

Earth’s Energy Source

• Sun’s Reactions• gravity of sun pulls hydrogen gases

together• with increased pressure, heat, &

density, hydrogen atoms fuse together

• Nuclear Fusion• 4H --> He + particles + ENERGY• Energy emitted as

• Electromagnetic Radiation• transmitted in various wavelengths

Radiation Emitted by the Sun

Electromagnetic Radiation (from Sun)

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Gammarays

Long wavelengths

Radio waves

0.01 nm 10 cm

Radi

ation

inte

nsity

Short wavelengths

X rays Ultraviolet

Visiblelight

0.4 μm 0.7 μm

Infrared

Terrestrial radiation(exaggerated about 100,000 ×)

Microwaves

0.1 nm 1 nm 10 nm 0.1 μm 1 μm 10 μm 100 μm 1 mm 1 cm

Wavelength

Solar radiation

Two ways to view this question:1. Absorption spectrum

wavelengths pigments absorb

2. Action spectrumwavelengths pigments use

What light is USED in photosynthesis?

Location of Photosynthesis

• What membranes are present in chloroplast?

• Why additional membranes?

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Lightenergy Energized

chlorophyllWater

H2OChlorophyll

Light-dependentreactions

High-energymolecules

Light-independentreactions

OxygenO2

Carbondioxide

CO2

Carbohydrates(CH2O)

Reactions of Photosynthesis

I. Energy-transduction reactions (light or photochemical reactions)

light energy chemical energy

ATP

NADPH

visible spectrum

II. Carbon-fixation reactions (dark or biochemical reactions)

carbon dioxide glucose

CO2 C6H12O6

simple compound complex organic compound

plants use photosynthesis to convert solar energy into energy rich biochemical compounds

Metabolism

• sum of the vast array of chemical reactions that occur in an organism

• step-by-step sequences: metabolic pathways

catabolism• oxidation reactions - loss of electrons• downhill reactions that release energy

anabolism• reduction reactions - gain of electrons• uphill reactions that require a net input of

energy

Cellular Energy

• Adenosine triphosphate (ATP)• energy release

• when terminal phosphate group is cleaved from molecule

• energy storage• when terminal phosphate group is replaced

Adenosine

P PP

ATPAdenosine

PP

ADP

energyP

Reaction Basics

• NADP • nicotinamide adenine dinucleotide

phosphate• primary electron acceptor in photosynthesis• temporary high-energy storage molecule

NADPH

NADP+

(reduced form)

(oxidized form)

oxidationreduction

I. Energy-transduction reactions

• transfer of electrons from light to reduce NADP+ into NADPH

• electron flow (also known as electron transport) creates ATP from ADP + P

• occurs within thylakoid membrane of chloroplast

Chloroplast Structure

I. Energy-transduction reactions

machinery for reactions1. photosystem

a. reaction-center molecule (a special chlorophyll a molecule) which receives electrons from light

b. electron acceptor which receives electrons from reaction center

c. antenna complex of 250-400 pigment molecules that gather and funnel light energy to the reaction center molecule

2. electron transport chain• molecules that move electrons from high to low energy

levels

3. ATP Synthase• proton pump to make ATP from ADP + P

(photophosphorylation)• runs off of the H+ gradient between thylakoid lumen and

stroma• also called chemiosmotic coupling

photosystem

electron-transport chain

ATP synthase complex

thylakoid membrane PSIPSII

ea eae - transport chain

ATP synthase

stroma

thylakoid lumen

e- transport chain

NADPH NADP+

2H+

ADP + P

ATP

2H20 2H++O2

photolysis

2e-+

2H+

I. Light-dependent reactions

I. Energy-transduction reactions

1. Electrons are produced from what compound in the energy-transduction reactions?

2. Electrons end up in which molecule?

3. Which photosystem is “first” in the energy-transduction reactions?

4. What is the name of the process that produces ATP?

5. ATP ends up in which location of the chloroplast?

6. Oxygen is produced in which location of the chloroplast?

7. Compared to the stroma, the thyllakoid lumen is __________ (acidic, basic)

I. Energy-transduction reactions

Uneven distribution of PS II and I. Characterize for me…

Noncylic electron flow and photophosphorylation

II. Carbon Fixation Reactions

• produce a complex organic compound (C6H12O6) from a simple compound (CO2)

CO2 C6H12O6

• occur in stroma of chloroplast • use energy from photochemical

reactions• are a set of cyclical reactions known as

the Calvin Cycle

Light-independent reactions

Calvin Cycle

carbon dioxideCO2

rubisco

1. Carbon Fixation

glucoseC6H12O6

2. Generation of PGAL

(reduction)3. Regeneration of carbon fixing

compound (RuBP)

Calvin cycle: initial carbon fixation

Calvin cycle: reduction of PGA into PGAL

Calvin cycle summary

Carbohydrate basics

made as PGAL in

photosynthesishexose

(glucose or

fructose)

transported as sucrose

stored as

starch

converted to

(glucose-fructose)

Fate of hexose

• converted to sucrose for transport• stored as starch for storage• made into cellulose for cell wall

material• used in respiration for energy• enters metabolic pathways to

generate other organic compounds

Connection of two photosynthetic reactions

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Lightenergy Energized

chlorophyllWater

H2OChlorophyll

Light-dependentreactions

High-energymolecules

Light-independentreactions

OxygenO2

Carbondioxide

CO2

Carbohydrates(CH2O)

Initial carbon fixation in Calvin cycle (C3)

• Enzyme• rubisco (ribulose 1,5-bisphosphate

carboxylase)• Substrate

• RuBP (ribulose 1,5-bisphosphate)• Product

• First stable product is PGA (3-phosphoglycerate)

Problems with the Calvin Cycle

1. photorespiration • efficiency of rubisco

• rubisco is better named:ribulose 1,5-bisphosphate carboxylase/oxygenase

• CO2 + RuBP --> 2 PGA• O2 + RuBP --> phosphoglycolic acid + PGA• phosphoglycolic acid leaves the Calvin Cycle

• How common is photorespiration?• atmosphere: 21 % O2, 0.036% CO2

• estimates that as much as 1/2 fixed C is lost

Photorespiration

Salvage of phosphoglycolate

Problems with the Calvin Cycle

2. Transpiration• CO2 in

• water vapor out

• Solutions:• additions to the basic plan

• C4 Photosynthesis

• CAM (Crassulacean Acid Metabolism)

C4 Photosynthesis – one solution

spatial separation of: • initial carbon fixation• Calvin cycle

C4 leaf anatomy

Initial carbon fixation

• Enzyme• PEP carboxylase

• Substrate• PEP (phosphoenolpyruvate)

• Product• Oxaloacetate (a 4C organic acid)• quickly converted to malate or aspartate

(other 4C organic acids)• NOT AN OXYGENASE!

• thus unaffected by O2 concentration

temporal separation of: • initial carbon fixation• Calvin cycle

CAM photosynthesis – another solution

Crassulacean Acid Metabolism

Crassulacean Acid Metabolism (CAM)

Comparisons of solutions:

Comparisons of Efficiency

C3 C4 CAMoptimum temp (°C) 15-2530-47 35

loss gH2O/gCO2 fixed 400-500 250-300 50-100

CO2:ATP:NADPH 1:3:2 1:5:2 1:6.5:2